A known method of manufacturing plastic bottles of thermoplastic synthetic resins includes the blow-molding of thin-walled containers from heavier parisons. These containers are generally made with rounded undersides since attempts to blow-mold flat-bottomed hollow bodies have not given satisfactory results in terms of stability against deformation. It has therefore already been proposed to assemble such bottles from two complementary members, i.e. a blow-molded body and a separately molded base cup, firmly bonded to each other.